To get the current username in wordpress using php you have write this custom code in functions.php
function getUser(){
$current_user = wp_get_current_user();
$userName = $current_user->display_name;
?>
<span><i class="fas fa-user"></i> <?php echo $userName ?></span>
<?php }
add_shortcode("username","getUser");
And then you can user this shortcode [username] anywhere that you want to output the user name.